1. Field of the Invention
The present invention relates to a method and a system for controlling an injection molding apparatus. More particularly, the present invention relates to a method and a system for advancing a control object using a driving mechanism and for stopping the control object at a target position.
2. Description of the Related Art
A control system for an injection molding apparatus which controls an advancing control object (such as an injection screw of an injection device, or a movable platen of a clamping device) is known. The known control system includes a driving mechanism having a servo motor. The servo motor is stopped upon the control object reaching a preliminarily set target position. Such a known control system is disclosed in Japanese Unexamined Patent Publication (Kokai) No. Heisei 5-138705.
FIG. 4 shows such a control system 40 of this type. In FIG. 4, an injection molding apparatus 41 has an injection device 42 and a clamping device 43. The clamping device 43 includes a servo motor 44. Upon clamping, a movable platen 45 is advanced from a forward motion start position by means of the servo motor 44 which is controlled by a control system. At this time, a position of the movable platen 45 is indirectly detected, on the basis of a number of revolutions of the servo motor 44, by means of an encoder 46.
An obtained position detected value Dxd is supplied to a first error detecting portion 47. The first error detecting portion 47 also receives a position command value Dxc. Then, a first error of the position detected value Dxd and the position command value Dxc is generated by the first error detecting portion 47.
The first error is converted into a converted speed command value Dvs via a position control portion 48. The position control portion 48 can include a compensation circuit or similar circuitry. The converted speed command value Dvs is received by a second error detecting portion 49. The position detected value Dxd is converted into a speed detected value Dvd by a speed converting portion 50. The speed detected value Dvd is also received by the second error detecting portion 49. Then, a second error of the speed detected value Dvd and the converted speed command value Dvs is generated by the second error detecting portion 49.
The second error is converted into a current command value Dic via a speed control position 51. The speed control portion 51 can include a compensation circuit or similar circuitry. The current command value Dic is applied to the servo motor 44 via a feedback control circuit 52.
The feedback control circuit 52 includes a current loop. It should be noted that the reference numeral 53 denotes a current detecting portion, 54 denotes a third error detecting portion and 55 denotes a current control portion. The current control portion 55 can include a compensation circuit or similar circuitry.
However, since a feedback control system for speed is incorporated in a feedback system for a position, drawbacks degrading accuracy and instability, depending upon the condition of a control object, can occur. Such drawbacks include instability of speed, failure of speed change at speed change position or hunting at a positioning position.
For example, when a load temporarily grows significantly of some reason, a lag in advancing of the control object occurs. The lag results in an error between the position detected value Dxd and the position command value Dxc becoming temporarily large. Therefore, the control object may exhibit unstable behavior, such as high speed motion upon resolving of the cause of the lag. Particularly, in a clamping operation, for the purpose protection of a mold, a clamping mode is transitioned from a high speed clamping to a low speed clamping before closing the mold. When the position error is significant, upon changing of speed, the clamping mode cannot be smoothly transitioned into the low speed clamping at the speed changing position. This results in clamping occurring during high speed and results in damage to the mold.